1. Field of the Invention
This invention relates to an optical reflecting device, especially to a reflectivity-tunable reflector, and more particularly relates to a reflectivity-tunable fiber optic Bragg grating reflector.
2. Description of Prior Art
Fiber optic Bragg gratings have been widely used in opto-electronic applications due to the advantages of low loss, high reliability, low cost, and direct connectability with fiber system, etc. For example, fiber gratings have been applied to the fabrication of sensors (stress, temperature, magnetic field, and accelerating velocity, etc), wavelength-stabilizing and narrowing elements for lasers, and filters, wavelength selectors, multiplexers and dispersion compensaters used in optical fiber communications.
The fabrication and the basic operation principles of fiber gratings are described below.
With the exposure to UV lasers through an interferometer or phase mask, the refractive index of the core of a fiber becomes periodically changed along the optical axis so as to form a fiber grating.
The most significant feature of a fiber optic Bragg grating is the relatively narrow bandwidth of its reflection spectrum. The reflection wavelength is determined by the grating period while the reflectivity is proportional to the depth of the refractive index modulation. Usually, the refractive index modulation can be as large as 0.001 with UV exposure of Ge-doped fiber and the reflectivity can be close to unity over the reflection band. Whatever the characteristics a fiber optic Bragg grating possesses, the reflectivity is fixed after it is fabricated. This property limits the application of a fiber optic Bragg grating. However, with an appropriate acoustic excitation method, the transverse vibration can induce the coupling between the core mode and the cladding modes; in this manner, the Bragg reflectivity of the fiber grating can be changed. The Bragg reflectivity can be varied from its original value down to almost zero. Hence, the device can serve as an acousto-optical fiber switch. Using an acoustic wave to control the behaviors of the mode coupling in optical fiber has been discussed.
In Opt. Lett., 19, 1964 (1994), Birks et al. disclose the use of an acoustic wave to excite the transverse vibration of a four-port taper fiber coupler for shifting the coupling frequency. In Opt. Lett., 21, 27 (1996), Yun et al. use an acoustic wave to achieve tunable filtering in a two-mode fiber coupler. Liu et al. use acoustic waves to excite the longitudinal vibration of a fiber optic Bragg grating for generating side bands of the reflection window in J. Lightwave Tech., 16, 2006 (1998). The spectral location and intensity of the side bands are controlled by the acoustic frequency and intensity, respectively. Although this mechanism can also be used for modulating reflectivity, it includes several drawbacks: 1) Fiber gratings of high Bragg reflectivity are required for significant side-band generation; 2) The acoustic intensity needs to be quite strong for efficient operation; 3) the proximity of the side bands to the original reflection window makes applications difficult. This makes the prior art complicated and difficult to manufacture.